5 2. “Packet” and “Frame” relationshipNetwork LayerNetwork LayerIn some cases, functions of error control and flow control are allocated in transport or other upper layer protocols and not in the DLL, but principles are pretty much the same.

8 list of the DLL requirementsFrame synchronization. Data are sent in blocks called frames. The beginning and end of each frame must be recognized.Flow control. The sending station must not send frames at a rate faster then the receiving station can absorb them.Error control. Any bit errors introduced by the transmission system must be checked & corrected.Addressing. On a multipoint line, such as a LAN, the identity of the two stations involved in a transmission must be specified.Link management. The initiation, maintenance, and termination of a data exchange requires a fair amount of coordination and cooperation among stations.

10 1. Unacknowledged connectionless serviceThe source machine send frames to the destination machine without having the destination machine acknowledged them.No logical connection is established beforehand or released afterward.If a frame is lost due to noise on the line, no attempt is made to detect the loss or recover from it in the DLL.This class of service is appropriate when the error rate is very low so that recovery task is left for solution to higher layers.It is also appropriate for real-time traffic, such as voice, in which late data are worse than bad data.Most LANs use unacknowledged connectionless service in the DLL

11 2. Acknowledged connectionless serviceIs more reliable.Still no logical connections used, but each frame sent is individually acknowledged.The sender knows whether a frame has arrived correctly.If it has not arrived within a specific time interval, it can be sent again.This service is useful over unreliable channels, such as wireless system.If the large packet is broken up into frames, If individual frames are acknowledged or retransmitted, entire packets get through much faster than unbroken frame that is lost, it may take a very long time for the packet to get through..

12 3. ACKed connection-oriented serviceThe service requires established connection between source/destination machines before data are transferred.Any frame sent over the connection is numbered, and the DLL guarantees that each frame sent, is received, and are received in the same order.With connectionless service, in contrast, it is possible that a lost acknowledgement causes a packet to be sent several times and thus received several times.When connection-oriented service is used, transfers go through 3 distinct phases:1. The connection is established and counters needed to keeptrack of which frames have been received and which oneshave not.2. One or more frames are transmitted and acknowledged.3. Connection is released, freeing up the variables - buffers and other resources used to maintain the connection.

14 Example is a WAN subnetConsisting of routers connected by point-to-point leased telephone lines.When a frame arrives at a router, the hardware checks it for errors, (Passes the frame to the DLL software which might be embedded in a chip on the network interface board).The DLL software checks to see if it is the frame expected,If so, gives the packet (contained the payload field) to the routing software.The routing software then chooses the appropriate outgoing line and passes the packet back down to the DLL software, which then transmits it.

15 Stop-and-wait ARQ Go-back-N ARQ Selective-reject ARQTechniques for error control are:Error detection.Positive Acknowledgment.Retransmission after time-out.Negative acknowledgment and retransmissionThese 4 mechanisms are all referred to as AutomaticReport reQuest (ARQ); the effect of ARQ is to turn an unreliable data link into a reliable one.Three standardized versions Of ARQ:Stop-and-wait ARQGo-back-N ARQSelective-reject ARQ

16 Link Layer Job (Cont) Flow Control:Two approaches are commonly used:Feedback-based flow control, the receiver sends back information to the sender giving it permission to send more data or at least telling the sender how the receiver is doing.“You may send me n frames now, but after they have been sent, do not send any more until I have told you to continue”.2. Rate-based flow control, the protocol has a built-in mechanism that limits the rate at which senders may transmit data. Since rate-based schemes are never used in the DLL

17 Elementary Data Link ProtocolsAssumptions:1). DLL and Network layer are independent processesthat communicate by passing messages back andforth trough the physical layer.2). a. Machine A wants to send a long stream of data tomachine B, using a reliable, connection-orientedservice.b. We will consider the case where B also wants tosend data to A simultaneously. A is assumed tohave a data ready to send.3). Machines do not crash.

18 Prtcl.1. Stop-and Wait ProtocolProtocol in which the sender sends one frame and then waits for an ACK: stop-and-wait.Δt (timeout); Damaged ACK; ACK0, ACK1.bidirectional information transfer.Half duplex physical channel.It is often the case that a source will break up a large block of data into smaller blocks and transmit the data in many frames, Reason:1. The buffer size of the receiver may be limited.2. The larger the transmission, the more error,With smaller frames, error are detected sooner, Smaller amount of data needs retransmission.3. On a shared medium, (LAN), it is usually desirable not to permit one station to occupy the medium for an extended period, as this causes long delay at the other sending stations.

19 Stop-and-Wait ARQ A B Frame lost A retransmits ACK1 lost A retransmitsTimeoutFrame lost A retransmitsFrame 0TimeoutACK1ACK1 lost A retransmitsFrame 0B discards duplicate frame

22 Prtcl.2. Simplex prtcl for Noisy Channel; Time-outData are transmitted in one direction only (simplex channel), that makes error. Frames may be either damaged or lost completely.Stop-and-wait protocol would work: adding a timer.a. The sender could send a frame, but the receiver would only send an ACK frame if the data were correctly received.b. If a damaged frame arrived at the receiver, it would be discarded.c. After a while the sender would time out and sends the frame again. This process would be repeated until the frame finally arrives intact.1-bit sequence number (0 or 1)

23 TCP Round Trip Time and TimeoutQ: how to set TCP timeout value?too short: premature timeout=unnecessaryretransmissionstoo long: slow reaction =timewastingQ: how to estimate RTT?SampleRTT: measured time from segment transmission until ACK receiptignore retransmissionsSampleRTT will vary, want estimated RTT “smoother”average several recent measurements, not just current SampleRTT

24 Fast Retransmit Time-out period often relatively long:long delay before resending lost packetDetect lost frame via duplicate ACKs.Sender often sends many frames back-to-backIf frame is lost, there will likely be many duplicate ACKs.If sender receives 3 ACKs for the same data, it presumes that frame after ACKed data was lost:fast retransmit: resend frame immediately, before timer expires

25 Protocol scenario:1. The network layer on A gives packet 1 to its DLL. The packet is correctly received at B and passed to the network layer on B.B sends an ACK frame back to A.2. The ACK frame gets lost completely. It just never arrives at all.3. The DLL on A times out. Not having received an ACK, it (incorrectly) assumes that its data frame was lost or damaged and sends the frame containing packet 1 again.4. The duplicate frame also arrives at the DLL on B perfectly and is randomly passed to the network layer there. If A is sending a file to B, part of the file will be duplicated (i.e., the copy of the file made by B will be incorrect and the error will not have been detected). In other words, the protocol will fail.

26 Sliding-Window Better idea is to use the Duplex Channel.Data frame from A to B are intermixed with the acknowledgment frames from B to A.By looking at the kind field in the header of an incoming frame, the receiver can tell whether the frame is data or ACK.Station B,-buffer space for n frames. Thus, B can accept n frames, and A is allowed to send n frames without waiting for any ACK.3-bit field, the sequence number can range from 0 to 7 0 through , from 0 to

27 Sliding-Window Pipeline 1 2 3 5 6 7 4 Frames already received1235674Frames already receivedWindow of frames thatmay be transmittedmay be acceptedFrameSequencenumberLast frametransmittedWindow shrinksfrom trailing edgeas frames are sentWindow expands fromleading edge as receivedacknowledgmentacknowledgedas frames are receivedleading edge as sent(a) Transmitter’s perspective(b) Receiver’s perspectivePipelinePiggybacking it is typically the technique of temporarily delaying outgoing ACKs so that they can be hocked onto the next outgoing data frame.Each data frame includes a field that holds the sequence number of that frame plus a field that holds the sequence number used for acknowledgment. Thus, if a station has data to send and an acknowledgment to send, it sends both together in one frame,

28 Pr.3. Example: Sliding-window protocolF0F1F2RR3F3F4F5F6RR7Source system ADestination system B(RR6); (RNR)piggybacking, it is typically the technique of temporarily delaying outgoing ACKs so that they can be hocked onto the next outgoing data frame. Each data frame includes a field that holds the sequence number of that frame plus a field that holds the sequence number used for acknowledgment.Maximum window size=7

30 Prtcl.5. A Protocol Using Go Back NFor efficiency of the bandwidth utilization:59 kbps satellite channel-500-msec round-trip delay.Sent 1000-bit frame. At t=0 msec-the frame has beenStarted and t=20 msec sent. Received t=270 msecframe fully arrived at the receiver; t=520 msec- ACK tothe sender; So, sender was blocked during 500/520 or96% of the time. 4 % of the bandwidth was used.The solution: the sender transmits up to w framesbefore blocking, instead of just 1 frame.The example, w should be at least 26. The sender begins sending Fr. 0 as before. Finishes sending 26 frames, at t=520 msec, the ACK for frame 0 will have just arrived. ACK arrive every 20 msec, (PIPLINING) so the sender always gets permission to continue when it needs it.

31 Pr.5.A Protocol Using Go Back N (Cont)If the channel capacity is b bits/sec, the frame size l bits, and the round-trip propagation time R sec, the time required to transmit a single frame is l/b sec. After the last bit of data frame has been sent, there is a delay of R/2 before that bit arrives at the receiver and another delay of at least R/2 for ACK to come back, for a total delay of R.In stop-and-wait the line is busy for l/b and idle for R, giving:Line utilization = l / (l+bR).=4%

36 Pr.7. HDLC Frame Format bit oriented; bit stuffing Flag 8 BitsAddress 8/16 BitsControl 8/16 BitsDataVariable LengthCRC 8/16 BitsCommendsResponseMasterSlaveFlag- synchronization.Address- address of the secondary station.Control- keep track of transmitted and received frames foracknowledgment and flow control.CRC- contains a checksum to ensure data integrity.Flag- used to signal the end of a frame, and possibly the startof the next frame.

37 Pr.7. High-Level Data Link ControlThree kinds of control fields:a. Informationb. Supervisoryc. Unnumbered.The protocol uses a sliding window, with 3-bit sequence number. Up to seven unacknowledged frames may be outstanding at any instant.SeqP/FNext1BitsTypeModifier(a)(b)(c)P-polling dataF-finished polling.(a)-nACK (reject)(b)-RNR(c)-Selective reject-retransmit specifiedFor ACK is used the number of the first frame not yet received (i.e.., the next frame expected).

38 Pr.7. High-Level Data Link ControlDifferent types of frames use different ACKs:ACKDefinitionUsedFrame witherrorProblems with the receiver shortage of buffersender’s window size is half or less the sequence spaceType1REJECTTransmission errorhas been detected2RECEIVE NOT READYAcknowledges allframes, but notincluding Next.Stop sending3SELECTIE REJECTRetransmissionof only the frame specified.

39 A Network Layer in the InternetLeasedLines toAsiaA U.S. backboneRegionalnetworkIP EthernetLANIP tokenRing LANA European backboneA1CDB2

42 Pr.8. PPP-The Point-to-point ProtocolPPP provides three features:A framing method that clearly determines the:end of one frame and the start of the next one,Error detection.A link control protocol for bringing lines up, testing them, negotiating options, and bringing them down again when they are no longer needed, This protocol is called LCP (Link Control Protocol). It supports synchronous and asynchronous circuits and byte-oriented and bit-oriented encodings.A way to negotiate network-layer options in a way that is independent of the network layer protocol to be used. The method chosen is to have a different NCP (Network Control Protocol) for each network layer supported.

43 Pr.8. PPP- Steps1. PC calls the provider’s router via a modem.Router2. The router’s modem has answered the phone and established a physical connection3. PC sends to the router a series of LCP packets in the payload field of one or more PPP framesATCThese packets and their responses select the PPP parameters to be used.Once the parameters have been agreed upon, a series of Network Control Protocol packets are sent to configure the network layer.Typically, the PC wants to run a TCP/IP protocol stack, so it needs an IP address.

45 Pr.8. PPP-Protocol fieldThe Protocol field’s job is to tell what kind of packet is in the Payload field.Codes are defined for LCP, NCP, IP, and other protocols.Protocols starting with a 0 bit are network layer protocols such as IP, IPX, OSI CLANP.Those starting with a 1 bit are used to negotiate other protocols. These include LCP and a different NCP for each network layer protocol supported.The default size of the protocol field is 2 bytes, but it can be negotiated down to 1 byte using LCP.

46 PPP-summaryPPP is a multiprotocol framing mechanism suitable for use over: Modems, HDLC,SONET, Other physical layers.It supports: Error detection, Option negotiating, Header compression.DLL converts the raw bit stream (from physical layer) into a stream of frames (for network layer).Various framing methods are used: character count, byte stuffing, and bit stuffing.Data link protocols can provide: 1. Error control to retransmit damaged or lost frames. 2.To prevent a fast sender from overrunning a slow receiver.The data link protocol also provide flow control.The sliding window mechanism is used to integrate error control and flow control in a convenient way